Morphological and hydrological characteristics of a recovering river landscape: Wollombi Brook, New South Wales

This study investigates the nature of hydrological changes in Wollombi Brook which have been inferred to be associated with changes in channel morphology and vegetation extent. This work addresses channel dynamics and hydrological characteristics of a recovering river landscape, having importance for flood prediction. Flood frequency analyses have been undertaken on Wollombi Brook from 1914 to the present. The flood record displays distinct multi-decadal periods of above and below average flood activity. The analyses spanned at least one Drought Dominated Regime (DDR) and one Flood Dominated Regime (FDR) per gauge and therefore provides insight into hydrological conditions under both regimes. The investigation revealed that over the past three decades channel morphologies on Wollombi Brook have remained relatively stable. Total annual flows have declined since 1991 along with a decline in the magnitude of infrequent of events. In addition to reductions in total annual flow, flood velocities have also reduced (for example Payne’s Crossing returned 0.5 m/s in the first DDR compared with 0.42 m/s in this DDR to date) over at least the period of the last DDR (1991 to present). There has been an increase in the number of 2-5 year ARI events and a decline in large scale events such as those v of 1949 and 1955. The results of this study therefore support the theory that Wollombi Brook is in a period of recovery following widespread channel change brought about by the 1949 flood and subsequent flood of 1955.